The Default Allocation % field is available under the Team object in CA PPM
Once you access this object, go to its View. You will find the staff member properties. For this section select the Fields. In the list of the fields, choose the properties for the Default Allocation field.
Ensure that the Value Required field is check marked.
This will make the Default Allocation mandatory.
Refer to the attached document for screenshot / steps
Let us know if this helps.
Regards,
Samik